Generally, in a fiber-reinforced sheet, high performance is developed by impregnating reinforcing fiber bundles with a resin and integrating the resin and the reinforcing fibers.
Examples of a resin with which reinforcing fiber bundles are impregnated include thermosetting resins and thermoplastic resins. The melt viscosity of high molecular weight thermosetting resins and general thermoplastic resins is high. It is not easy to impregnate reinforcing fiber bundles with a resin having high melt viscosity. Therefore, it is not easy to mold a fiber-reinforced sheet into various shapes, and it is not easy to mold particularly a large-sized fiber-reinforced sheet.
A fiber-reinforced plastic molded article having a curved shape is disclosed in the following Patent Literature 1. This molded article is a laminate of at least three or more layers comprising continuous fiber-reinforced sheets having fiber bundles. In this molded article, a nonwoven fabric sheet is sandwiched in the second layer from the outermost layer constituting a design surface. This molded article is curved in the direction orthogonal to the orientation direction of the continuous fiber bundles in the outermost layer. This molded article is used, for example, as a component or an enclosure portion of a personal computer, OA equipment, a cellular phone, or the like. In Patent Literature 1, the molded article is relatively small, and a windmill blade is not particularly described as an application of the molded article.
Conventionally, large-sized molded bodies are used for windmill blades, aircraft, ships, and the like. High strength, light weight, and high durability are required of windmill blades used for wind power generation.
A windmill blade in which a spar cap that is a main strength member of the blade is disposed between outer cover layers or on the inside is disclosed in the following Patent Literature 2. The above spar cap is molded separately from the blade (windmill blade body). The windmill blade is obtained by disposing the above spar cap on a mold together with dry fibers for the above outer cover layers and a sandwich core material and impregnating them with a resin with vacuuming.